The cell surface or plasma membrane participates in many processes that are integral to the well-being of the cell, including cellular recognition and adhesion. In addition, plasma membranes of embryonic muscle cells are involved in cell fusion of mononucleate myoblasts to form multinucleated cells which develop into adult muscle fibers. In order to elucidate the molecular mechanism of muscle cell recognition and fusion, it is necessary to have a better understanding of the composition, metabolism and organization of the muscle cell plasma membranes. Fractions enriched in the plasma membrane markers, Na, K ion-ATPase and acetylcholine receptors, have been isolated from embryonic muscle cells growing in cell culture. The distributions of two other plasma membrane markers, phosphodiesterase I and leucyl beta-naphthylamidase, however, do not coincide with the distributions of the Na, K ion-ATPase and acetylcholine receptors. The first part of this research proposal involves determining if the NA, K ion-ATPase and acetylcholine receptors are evenly distributed throughout the entire surface membrane or if the plasma membrane consists of heterogeneous regions which are fragmenting into vesicles of different density and enzyme content. The approach will be to (a) compare the distributions upon subcellular fractionation of the enzymes listed above with additional plasma membrane markers, including radioactive surface labels, and (b) prepare plasma membranes as large sheets representative of the entire surface. The second part of the research proposal deals with the analysis of the composition of the plasma membranes from developing muscle cells, and includes analyses of the protein, glycoprotein and glycolipid compositions. Plasma membranes from muscle cells at several developmental stages will be analyzed to determine which components of the membrane are involved in cellular recognition and fusion.